4 - Stroke Marine Diesel Engine Construction

THE ENGINE FRAME: The engine frame of a modern 4 stroke medium speed diesel can be produced as a single casting or fabricated from cast steel sections and steel plates welded together.
With this design, there is no separate bedplate, frame and entablature as with a 2 stroke slow speed engine.

The photograph above shows the frame of an engine with the liners and crankshaft in place.

An alternative method of construction consists of a separate bedplate, which is bolted to an entablature which holds the underslung crankshaft.

When the Crankshaft is underslung, the load on the bearing caps is transferred back to the frame by the use of tie bolts.

Note the use of the side tie bolts - which locate the bearing cap, and prevent sideways movement.



THE CONNECTING ROD: The connecting rod in a medium speed 4 stroke engine is subject to an inertia whip loading due to the mass of the con rod swinging about the piston pin. (Because of the lower speed of a 2 stroke engine, the whip loading is not large enough to influence the design of the con rod) Added to this, the inertia loads due to the mass of the reciprocating parts cause a stress reversal from high compressive stress (during power and compression stroke) to a low tensile stress between the exhaust and inlet strokes.

This loading of the rod influences its design, and to withstand the loading described above, connecting rods are often forged from a manganese molybdenum steel in an I or H section which reduces its mass from one made of round section steel (and thus reduces the whip loading) while maintaining strength.

Because of the large diameter of the crankpin to increase bearing area and decrease bearing load, the width of the bottom end of the con rod is greater than the diameter of the cylinder liner.

So that the piston can be withdrawn from the liner, 3 different designs are used:
• The con rod can be fitted with a marine palm.
• The con rod can be split in two parts.

• The bottom end can be split obliquely. Serrations are used to locate the two halves relative to one another.

The advantage of using a vee engine is that the overall length of the engine is reduced for a given power output.

If a normal bottom end arrangement is used then the con rods must be placed side by side which means the opposite cylinders are offset. The crankpins must be long enough to accommodate two bottom ends side by side, and of large enough diameter to resist bending. The increased length of the crankshaft means a longer engine.

Bottom End Bolts
Because of the stress reversal mentioned above, bottom end bolts have a limited life. This varies from engine to engine, but is generally around 12-15000 hours. If a bottom end bolt was to fail in operation, then the results would be disastrous.

Bottom end bolts should be treated with care when removed from the engine during overhauls. They should be inspected for any damage to the surface from which a crack could start. This damage could be due to corrosion (water in LO) or because of incorrect handling.

THE CYLINDER LINER: The cylinder liner is cast separately from the main cylinder frame for the same reasons as given for the 2 stroke engine which are:
• The liner can be manufactured using a superior material to the cylinder block. While the cylinder block is made from a grey cast iron, the liner is manufactured from a nodular cast iron alloyed with chromium, vanadium and molybdenum. (cast iron contains graphite, a lubricant. The alloying elements help resist corrosion and improve the wear resistance at high temperatures.)
• The cylinder liner will wear with use, and therefore may have to be replaced. The cylinder jacket lasts the life of the engine.
• At working temperature, the liner is a lot hotter than the jacket. The liner will expand more and is free to expand diametrically and lengthwise. If they were cast as one piece, then unacceptable thermal stresses would be set up, causing fracture of the material.
• Less risk of defects. The more complex the casting, the more difficult to produce a homogenous casting with low residual stresses.

Modern liners employ bore cooling at the top of the liner where the pressure stress is high and therefore the liner wall thickness has to be increased. This brings the cooling water close to the liner surface to keep the liner wall temperature within acceptable limits so that there is not a breakdown in lubrication or excessive thermal stressing. Although the liner is splash lubricated from the revolving crankshaft, cylinder lubricators may be provided on the larger engines.

On the example shown above, the lubricator drillings are bored from the bottom of the liner circumferentially around the liner wall. Another set of holes are drilled to meet up with these vertically bored holes at the point where the oil is required at the liner surface.
Other engines may utilise axial drillings as in a two stroke engine.

Where the cooling water space is formed between the engine frame and the jacket, there is a danger that water could leak down and contaminate the crankcase if the sealing O rings were to fail. As a warning, "tell tale" holes are led from between the O rings to the outside of the engine.

Modern engines tend not to use this space for cooling water. Instead a separate water jacket is mounted above the cylinder frame. This stops any risk of leakage of water from the cooling space into the crankcase (or oil into the cooling water space), and provides the cooling at the hottest part of the cylinder liner.

Note that the liner shown above is fitted with a fireband. This is sometimes known as an antipolishing ring. It is slightly smaller in diameter than the liner, and its purpose is to remove the carbon which builds up on the piston above the top ring. If this carbon is allowed to build up it will eventually rub against the liner wall, polishing it and destroying its oil retention properties.

The liner must be gauged regularly to establish the wear rate and check that it is within manufacturers tolerances. The wear rate for a medium speed liner should be below 0.015mm/1000hrs. Excessive wear is caused by lack of lubrication, impurities in fuel air or Lubricating oil, bad combustion and acid attack.

THE PISTON: Pistons for medium speed trunk piston engines which burn residual fuel are composite pistons; i.e the crown and the skirt are made of different materials.

The crown is a heat resisting steel forging which may be alloyed with chromium, molybdenum and nickel to maintain strength at high temperatures and resist corrosion. It is dished to form a combustion chamber with cutouts to allow for the valves opening. The topland (the space between the top ring and the top of the piston) may be tapered to allow for expansion being greater where the piston is hottest.

The skirt can either be a nodular cast iron or forged or cast silicon aluminium alloy. Aluminium has the advantage of being light, with low inertia, reducing bearing loading. However because aluminium has a higher coefficient of expansion than steel, increased clearances must be allowed for during manufacture. This means that the piston skirt clearance in the liner is greater than that for cast iron when running at low loads. The skirt transmits the side thrust, caused by the varying angularity of the con rod, to the liner. Too big a clearance will cause the piston to tilt.

The piston pin for the con rod small end bearing is located in the piston skirt. The piston pin floats in the piston skirt and is located in place by circlips. Depending on the material used for the skirt (esp. cast aluminium), a bushing may be used for the pin.

The piston rings may be located in the crown or in both crown and skirt. Normally, the rings are chrome plated or plasma coated to resist wear. Because the liner is splash lubricated, an oil scraper (oil control) ring is fitted to the piston skirt.

The piston is oil cooled. This is achieved by various means; The simplest is for a jet of oil to be directed upwards from a hole in the top of the con rod onto the underside of the crown. A more efficient method is to use an oil catcher as shown in the picture above. This directs oil into the cooling spaces on the underside of the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is unusual for the oil return temperature to be monitored (unlike the 2 stroke slow speed crosshead engine, where both temperature and quantity are monitored).

Some engines are fitted with one piece pistons manufactured from either cast iron or silicon alloy aluminium . These cannot be used with residual fuel, because the higher temperatures causes burning of the piston crown. Aluminium also suffers from carbon build up above 300 deg. C.

THE CYLINDER HEAD: Cylinder heads for 4 stroke engines are of a complex design. They have to house the inlet and exhaust valves, the fuel injector, the air start valve, relief valve and indicator cock. The passages for the inlet air and exhaust gas are incorporated, as are the cooling water passages and spaces.

Normally manufactured from spheroidal graphite or nodular cast iron which is easy to cast. Although not as strong as cast steel, which is difficult to cast into complex shapes due to its poor fluidity, it maintains a reasonable strength under load. Adequate cooling is essential to prevent thermal fatigue due to uneven expansion throughout the casting, and bore cooling has been introduced along with cooling spaces to ensure effective cooling of the "flame plate" (the underside of the cylinder head which forms the top of the combustion chamber).

Cracking of cylinder heads can occur due to poor cooling causing thermal fatigue. Poor cooling can be the result of scale build up within the cooling spaces due to inadequate water treatment. Overloading of the unit causing high peak pressures is also a cause as is incorrect tightening down of the cylinder head. Cracking normally occurs between the valve pockets and/or cooling water spaces. Cracked cylinder heads can be repaired by specialised welding; but this must be done under the guidance and with authorisation from the classification societies.

THE VALVES AND ROCKER GEAR: The 4 stroke marine diesels used for main or auxiliary power on board ship will have multiple inlet and exhaust valves fitted to the cylinder heads. On the medium speed engines this normally takes the form of two inlet and two exhaust valves per unit. The reasons for this are as follows:

• The area of the valve openings must be large enough to provide for an efficient gas exchange process. If the area is too small then not enough air will be pushed/drawn into the cylinder during the induction stroke, and on the exhaust stroke the engine will be doing work pushing the exhaust gas out of the cylinder. 
• The cylinder head must accommodate inlet and exhaust valves, so unlike a two stroke engine, one large central exhaust valve is not possible. 
• If the valves are too large, then the strength of the cylinder head will be compromised. 
• Keeping the exhaust valve temperature within acceptable limits is of paramount importance. It is easier to cool a smaller valve. 
• The moving parts and springs are of smaller proportions reducing the inertia of the parts and the power demand on the engine. 
• A symmetrically designed strong cylinder head  is achieved. 

Exhaust valves are subject to arduous conditions, and require regular overhaul. To aid this, exhaust valves are often fitted in separate cages. This allows the exhaust valve to be changed and overhauled without removing the cylinder head. The cages have water cooling passages connected to the cylinder head cooling water.

                                                            Exhaust valve in a Cage

The cage is of cast steel. The cooled seats are made from a heat resistant molybdenum steel which may be stellite faced. The exhaust valve may be of a similar material or of a nimonic alloy.
Inlet valves are subject to much less arduous conditions and are not usually fitted in separate cages.
Two different sized springs are fitted to aid positive closing of the valves. The reason for fitting two springs are that if one fails, the other will prevent the valve dropping down into the cylinder. The two springs have different vibration characteristics, so the incidence of resonance is reduced. (resonance is where two items vibrate at the same frequency thus the amplitude of the vibration is amplified.)

Exhaust valves are designed to rotate in service. The reasons for this are to prevent uneven temperatures  so it does not distort and leak by, and to help dislodge any build up of deposits on the valve and seat which may prevent the valve closing properly and lead to "hammering" of the seating faces. A mechanical method is generally used, and this is either the "rotocap" or the "turnomat". Winged rotators or spinners as used on the 2 stroke engine exhaust valves can also be used, but this entails using a ball bearing race between the spring carrier and the cover.

Burning Out of Exhaust Valves:
Once an exhaust valve does not seat correctly, the high pressure burning gas will pass across the faces of the valve and seat during the power stroke. This will cause the temperature of the valve and seat to rise in this area, weakening the material and distorting the surfaces. The velocity of  the burning gas will erode the surface, allowing more gas to leak by. The temperature of the valve in this area will rise further, leading to further burning and greater distortion. The first indication of a valve burning out will be a rise in the exhaust temperature, which will rapidly increase together with a loss of power from the unit.

Vanadium slag deposits which occur at temperatures above 540º C cause corrosion of the valve surfaces which can lead to exhaust gas blow by. This is combated by effective cooling and  the use of suitable materials (stellite and nimonic alloys).

Rocker Gear:
Most medium speed four strokes use push rods and rocker gear to open and shut the valves at the correct time. Operated by cams, mechanically timed to the crankshaft, the pushrods transmit the motion to the rocker gear, which pushes the valves open at the correct time.

                                                          Master and Slave  

                                                            Yoke

Because there are two of each valve mounted in the cylinder head, the rocker gear must operate both valves simultaneously. Various methods are used including master and slave arrangements and yoke.

Rocker or Tappet Clearances:
Rocker or Tappet clearances refer to the clearance between the top of the valve spindle and the rocker arm. It is to ensure that the valve closes properly when it expands as it gets to operating temperature. Clearances are set according to manufacturers instructions, but usually done with the engine cold, and with the push rod follower on the base circle of the cam (one way of ensuring this is to turn the unit being adjusted to TDC on the power stroke).

If the clearance is too small, then not only is there a chance that the valve will not close properly when it comes up to temperature, but it effectively will open early and close late.
Conversely if the clearance is too large, then the valve will open late and close early

THE CRANKSHAFT: The Crankshaft for a medium speed 4 stroke diesel engine is made from a one piece forging. First the billet of 0.4% carbon steel is heated in a furnace It is then moved to the forging presses where the crankshaft throws and flanges are formed.

The crankshaft is locally heated to a white heat where the webs are desired to be formed. The crankshaft is then compressed axially to form the start of the webs. Sets of hydraulic presses are then used to form the crankpin journal and webs.

This method of forging gives the crankshaft continuous grain flow. This is where the grain structure follows a path parallel to and along the journal, bends round along the line of the web, round through the crankpin, and back down the second web before turning again to follow the journal. Continuous grain flow gives the crankshaft better fatigue resistance.

The forgings are then machined, stress relieved, and the radii at the change of section cold rolled. If the crankshafts are to be surface hardened they are made of a steel alloy known as nitralloy (a steel containing 1.5%Cr, 1% Al and 0.2% Mo). The crankshaft is heated to 500ºC in ammonia gas for up to 4 days. The nitrogen dissociates from the ammonia gas and combines with the chromium and aluminium to form hard nitrates at the surface. The molybdenum refines the grain structure at the still tough core.

At the change of section between journal and web and web and crankpin, fillet radii are machined so there is not a sharp corner to act as a stress raiser. These radii are cold rolled to remove machining marks, harden the surface and to induce a residual compressive stress, again to increase fatigue resistance. Re-entrant fillets are sometimes employed; This allows for a shorter crankshaft without compromising on bearing length.

Oil Holes in Crankshafts: Unlike the crankshafts for slow speed 2 stroke crosshead engines, which lubricate the bottom ends by sending the oil down the con rod from the crosshead, the crankshaft for the medium speed trunk  engine must have holes drilled in it so that oil can travel from the main bearing journals to the crankpin and then up the con rod to lubricate the piston pin and cool the piston. If the surface finish of the holes is not good, then cracks can start from the flaws.At the exit points on the crankpin, the holes must be smoothly radiused. So that the crankshaft strength is not compromised the holes should be positioned horizontally when the crank is at TDC.

THE FUEL PUMP: Medium speed four stroke engines are equipped with jerk type fuel pumps, one for each cylinder. A plunger operated by a cam reciprocates in a barrel.

The plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The  barrel is located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor.

As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should therefore be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, then no injection will take place and the engine will stop.

More common are pumps with two helices (and thus two no load grooves) diametrically opposite each other. This gives a balanced plunger. (shown below)

The plunger is machined to very fine tolerances, as is the matched barrel in which it reciprocates. Wear due to abrasive particles in the fuel will mean that the pump will take longer to build up the injection pressure required.

Wear due to erosion also takes place on the top edge of the plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.

THE FUEL VALVE (FUEL INJECTOR): The fuel is delivered by the fuel pumps to the fuel injectors or fuel valves. For the fuel to burn completely at the correct time it must be broken up into tiny droplets in a process known as atomisation. These tiny droplets should penetrate far enough into the combustion space so that they mix with the oxygen.  The temperature of the droplets rise rapidly as they absorb the heat energy from the  hot air in the cylinder, and they ignite and burn before they can hit the relatively cold surface of the liner and piston.

Fuel injectors achieve this by making use of a spring loaded needle valve. The fuel under pressure from the fuel pump is fed down the injector body to a chamber in the nozzle just above where the needle valve is held hard against its seat by a strong spring. As the fuel pump plunger rises in the barrel, pressure builds up in the chamber, acting on the underside of the needle as shown. When this force overcomes the downward force exerted by the spring, the needle valve starts to open. The fuel now acts on the seating area of the valve, and increases the lift.

As this happens fuel flows into the space under the needle and is forced through the small holes in the nozzle where it emerges as an "atomised spray".

At the end of delivery, the pressure drops sharply and the spring closes the needle valve smartly.

The pressure at which the injector operates can be adjusted by adjusting the loading on the spring. The pressure at which the injectors operate vary depending on the engine, but can be as high as 540bar.

Fuel injectors must be kept in good condition to maintain optimum efficiency, and to prevent conditions arising which could lead to damage within the cylinder. Injectors should be changed in line with manufacturers recommendations, overhauled and tested. Springs can weaken with repeated operation leading to the injector opening at a lower pressure than designed. The needle valve and seat can wear which together with worn nozzle holes will lead to incorrect atomisation and dribbling.